Method for fabricating a low resistance TMR read head

ABSTRACT

A method is provided for forming a magnetoresistive read head with an MTJ configuration having an ultra-thin tunneling barrier layer with low resistance and high breakdown strength. The barrier layer is formed by natural oxidation of an ultra-thin (two atomic layers) Al or Hf—Al layer deposited on an electrode whose surface has first been treated to form an oxygen surfactant layer, the oxygen within the surfactant layer being adsorbed within the ultra-thin layer to produce a uniform and stable Al 2 O 3  stoichiometry (or HfO stoichiometry) in the tunneling barrier layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the use of magnetic tunnel junction (MTJ)configurations to form tunneling magnetic read heads (TMR read heads).In particular, it relates to the formation of a low resistance MTJ byuse of an under-oxidized tunneling junction layer that is naturallyoxidized and formed on an electrode with an oxygen surfactant layer.

2. Description of the Related Art

The magnetic tunnel junction (MTJ) basically comprises two electrodes,which are layers of ferromagnetic material, separated by a tunnelbarrier layer, which is a thin layer of insulating material. One of theelectrodes, the “pinned” electrode, has its magnetic moment fixed inspatial direction by an adjacent pinning layer (typically a layer ofantiferromagnetic material). The other electrode, the “free” electrode,has a magnetic moment that is free to move in the presence of externalmagnetic fields. Although the tunnel barrier layer between the free andpinned electrodes is an insulator, it is sufficiently thin so that thereis a small but finite probability for charge carriers (typicallyelectrons) to cross the layer by means of quantum mechanical tunneling.Thus, a probability dependent current can flow between the free andpinned electrodes. The tunneling probability is spin dependent,depending on the orientation of the conduction electron spin relative tothe magnetization direction of each of the ferromagnetic layers. Thus,if these magnetization directions are varied relative to each other, thetunneling current will also vary as a function of the relativedirections for a given applied voltage. When used as a read head, themagnetization of the free electrode varies with the external magneticfield of the recorded medium and the resulting change in current issensed by appropriate circuitry. The MTJ is already being successfullyused as a read head. Fontana, Jr. et al (U.S. Pat. No. 5,901,018)disclose a MTJ magnetoresistive read head with a free magnetic layerthat also functions as a flux guide to direct magnetic flux from therecording medium to the tunnel junction. The tunnel barrier layer taughtby Fontana is formed by depositing and then plasma oxidizing a 0.5-2 nmAl layer to form a layer of Al₂O₃. Wang et al. (U.S. Pat. No. 6,462,541B1) discloses sensing arrangment formed of a plurality of magnetic fieldsensors, each one generally formed of two magnetically permeable filmsseparated by an intermediate layer of non-magnetic material. Mao et al.(U.S. Pat. No. 6,411,478 B1) teaches the formation of an MTJ type readsensor whose layer structure is horizontal rather than vertical so as toachieve a thinner overall fabrication.

For a read head to operate successfully with recording densities greaterthan 100 Gb/in², it should have the following properties:

-   -   a) Very low junction resistance R for a given junction area A,        (the product, RA, being less than or equal to 1 Ωμm²)    -   b) High ratio of dR/R (>10%)    -   c) High dielectric breakdown voltage (>0.5 volts)        For a typical junction area: A=0.01 μm², R must be approximately        100 Ω for the tunneling magnetoresistive junction (MTJ). For        reference, R for a typical GMR sensor (not an MTJ) of this size        is approximately 50 Ω.

It is known that ultra-low junction resistance can be obtained only withan ultra-thin tunneling barrier. Al₂O₃ is known to be an insulator witha relatively wide band gap even when formed as an ultra-thin layer lessthan two atomic layers thick (<6 angstroms). Tunneling junctions usingthis material have been formed by a process of naturally oxidizing athin layer of aluminum. In this regard, Gallagher et al. (U.S. Pat. No.6,226,160) teach a method of thermally oxidizing a layer of Al between5-9 angstroms thick deposited on a fixed ferromagnetic layer, which doesnot oxidize the surface of the fixed ferromagnetic layer on which theoxide layer is formed. HfAlOx has also been shown to have very lowresistance at thin layer formations and the natural oxidation of a thinlayer of HfAl has produced an RA as low as 0.6 Ωμm² and dR/R ofapproximately 10%. Of the three requirements, a), b) and c), mentionedabove, only adequate breakdown voltage has not yet been obtained and isnot disclosed in the cited prior art.

SUMMARY OF THE INVENTION

A first object of this invention is to provide a novel MTJ read-headcapable of reading recorded densities exceeding 100 Gb/in².

A second object of this invention is to provide an MTJ read-head havingan ultra-low junction resistance, R, where RA (resistance times junctionarea) is less than 1 Ωμm².

A third object of this invention is to provide such an MTJ read headhaving a high magnetoresistive ratio, dR/R, which exceeds 10%.

A fourth object of this invention is to provide such an MTJ read headwherein the tunneling junction has a high breakdown strength.

These objects will be achieved by a fabrication method that forms anultra-thin tunneling barrier layer of an Al oxide or an Hf/Al alloyoxide on a bottom electrode (pinned electrode) that has first beentreated by an oxygen ambient to form an oxygen surfactant layer. Theimportance of such surfactant layers in the improvement of specularreflection scattering at interfaces of GMR spin valve structures hasbeen demonstrated by W. F. Egelhoff et al., “Oxygen as a surfactant inthe growth of giant magnetoresistance spin valves,” J. Appl. Phys.82(12), 15 Dec. 1997 p. 6142. We have found that the prior surfactantformation is also essential to creating the thin barrier layer that hashigh breakdown voltage in the TMR structure. If the layer is justnaturally oxidized, but not formed on a surfactant layer, it will have atendency to form an aluminum oxide without the proper Al₂O₃stoichiometry and stability to insure high breakdown voltage. The lowbreakdown strength of such layers which are not formed in accord withthe method of the present invention have already been reported byJianguo Wang et al. “Continuous thin barriers for low-resistancespin-dependent tunnel junctions,” J. Appl. Phys. (93)10, 15 May 2003, p.8367. In the present invention it is found that by combining naturaloxidation of an ultra-thin layer which has been formed on an oxygensurfactant layer, the requisite Al₂O₃ stoichiometry and barrier layerproperties are obtained because both the natural oxidation and theoxygen surfactant layer provides a source of oxygen for the aluminumlayer. It is determined experimentally that upon depositing an Al layeron an electrode having an oxygen surfactant layer containing less thanan oxygen monolayer, a high quality Al₂O₃ is immediately obtained at theinterface. During the subsequent natural oxidation to complete theoxidation of the Al layer, the Al₂O₃ already formed serves as an oxygendiffusion layer to prevent oxygen from diffusing into the bottomelectrode.

More particularly, two structures and methods of fabricating them areprovided. Structure I) uses an oxidized aluminum layer of approximately5.75 angstroms thickness, while structure II) uses an oxidized Hf—Allayer wherein the Hf is between approximately 1.5 and 2.5 angstromsthick and the Al is between approximately 3 and 4.5 angstroms thick.

-   I) NiCr/MnPt/CoFe(10%)/Ru/CoFe(10%)-10Å NiFe(60%)/OSL/5.75Å    Al/NOX/10Å NiFe(60%)-NiFe(20%)/Ta or:    NiCr/MnPt/CoFe(10%)/Ru/CoFe(25%)-10Å NiFe(60%)/OSL/5.75Å Al/NOX/10Å    NiFe(60%)-NiFe(20%)/Ta-   II) Ta/NiCr/MnPt/CoFe(10%)/Ru/CoFe(10%)-10Å    NiFe(60%)/OSL/Hf—Al/NOX/10 ÅNiFe(60%)-NiFe(20%)/Ta

The formation of the structures I) and II) will be further discussed andexplained in the context of the description of the preferred embodimentspresented below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-d are schematic illustrations of the processing stages leadingto the read-head configuration shown above as I).

FIG. 2 is a schematic illustration showing how the process steps of FIG.1 b would be changed to produce the device in II).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The two preferred embodiments of the present invention (I and II above)are each a method of forming a magnetoresistive read head in a magnetictunneling junction (MTJ) configuration in which an ultra-thin barrierlayer is formed with a more stable stoichiometry that gives it a highbreakdown voltage together with an ultra-low junction resistance. In thefirst preferred embodiment the barrier layer is formed by the naturaloxidation (NOX) of an ultra-thin aluminum layer deposited on asub-atomic monolayer of oxygen (an oxygen surfactant layer (OSL)) thatwas formed on a bottom electrode surface. The combination of the OSLbeneath the Al layer and the NOX of the Al layer produces an oxidized Allayer with Al₂O₃ stoichiometry. In the second preferred embodiment, theread head is formed using a naturally oxidized HfAl layer.

Referring first to FIG. 1 a, there is shown a cross-sectional view aninitial step in the formation of an MTJ device designed to efficientlyachieve the objects of the present invention while using a naturallyoxidized Al layer as a tunneling barrier layer. There is seen asubstrate (10), which in a preferred embodiment of this invention couldcontain structures to allow electrical connection to a bottom electrodeof the MTJ device. On the substrate is then formed a bottom electrode(20), which includes a magnetically pinned layer (22) and whichcomprises a Ta/NiCr(40%) seed layer (21), wherein the Ta layer providesan advantageous crystalline growth direction for the NiCr, and anMnPt/CoFe(10%)/Ru/CoFe(10%)-NiFe(60%) synthetic antiferromagnetic (SyAF)pinned layer formed on the seed layer, wherein the antiferromagneticmaterial MnPt (22) provides pinning of two antiferromagnetically coupledferromagnetic layers CoFe(10%) (24) and the composite layer CoFe(10% or25%)-NiFe(60%)(26 and 27) coupled by a Ru layer (25). The indicatedpercentages represent atom percentages of the Fe and Cr in the materialcomposition of the layers. The NiCr(40%) seed layer (21) is formed to athickness between approximately 30 and 40 angstroms. The CoFe(10%) layer(24) contacting the MnPt layer (22) is formed to a thickness of betweenapproximately 20 and 25 angstroms. The Ru coupling layer (25) is formedto a thickness between approximately 7 and 8 angstroms, the CoFe layer(26) formed on the Ru layer is formed to a thickness betweenapproximately 20 and 25 angstroms. A layer of NiFe(60%) (27) ofthickness between approximately 5 and 15 angstroms, with 10 angstromsbeing preferred, is formed on the CoFe layer (26). It is this NiFe layer(27) which is treated in an oxygen ambient to form an oxygen surfactantlayer (OSL) approximately less than 1 atomic monolayer in thickness (30)on its upper surface. The OSL on the upper surface of (27) is indicatedas the shaded layer (30). Our experiments suggest that forming theadditional NiFe layer is particularly advantageous for achieving theobjectives of the present invention because a surfactant monolayer (orsub-monolayer) of oxygen adsorbed on a NiFe layer is more effective forthe purposes of the present invention than a surfactant layer of oxygenadsorbed in a CoFe layer. This is because the lower binding energy ofoxygen with Fe (as compared to its binding energy with Co) allows theoxygen to be more easily incorporated into the subsequently deposited Allayer and to achieve the stable stoichiometry discussed below. A methodfor forming an OSL on an exposed surface that achieves the objectives ofthe invention is as follows:

-   -   1. After NiFe(60%) (27) deposition on CoFe layer (26), the        entire fabrication is moved into a separate vacuum chamber.    -   2. The chamber is fed with a small amount of oxygen (approx.        0.02 sccm) to obtain an oxygen pressure of about 2×10⁻⁷ torr.    -   3. The NiFe(60%) surface is exposed to the oxygen for about 100        s., forming less than a monolayer of oxygen on the surface.

Referring next to FIG. 1 b, there is shown the fabrication of FIG. 1 awherein an ultra-thin (2 atomic layers) layer of aluminum (40), betweenapproximately 5 and 6 angstroms thick, with approximately 5.75 angstromsbeing preferable, is formed on the oxygen surfactant layer (30). Theoxygen in the surfactant layer is immediately incorporated within theatomic structure of the Al, creating an Al₂O₃ stoichiometry at theinterface of the layers (35). The region of oxygen adsorption and Al₂O₃stoichiometry is shown as shaded (35).

Referring next to FIG. 1 c, there is shown the Al layer ((40) in FIG. 1b) after a natural oxidation process (NOX) to complete the formation ofan aluminum oxide tunneling barrier layer (50) with approximately Al₂O₃stoichiometry throughout its thickness. During the natural oxidation,the interface portion of the barrier layer ((35) in FIG. 1 b) acts as abarrier to oxygen diffusion into the pinned electrode (20). The NOXprocess occurs in the same chamber as the OSL formation, except that theoxygen is present at higher pressure (between 0.1 torr and 1 torr) andthe surface is exposed for corresponding times (between approximately500 and 200 seconds), with the shorter time being required for thehigher oxygen pressure. The OSL layer ((30) in FIG. 1 b) is no longerindicated by shading, because ideally the oxygen in that layer wouldhave been adsorbed into the aluminum oxide layer (50) to complete theAl₂O₃ stoichiometry. Similarly, the interfacial region ((35) in FIG. 1b) has now merged with the NOX region and layer (50) is, therefore,shown uniformly shaded.

Referring next to FIG. 1 d, there is shown schematically the fabricationof FIG. 1 c wherein there has been the formation of an upper electrode,which is a ferromagnetic free layer (60), on the tunneling barrier layer(50). Note that layer (50) is now shown with uniform shading to indicatethe uniform Al₂O₃ stoichiometry that has been achieved by the naturaloxidation and the absorption of the oxygen in the OSL. The free layer(60) is a layer of NiFe(60%)-NiFe(20%) having a total thickness betweenapproximately 30 and 50 angstroms, with between approximately 5-10angstroms of the NiFe(60%) being preferred. A capping layer of Ta (70)is formed on the free layer. It is understood that the fabricationdescribed in the foregoing description will be formed between upper andlower NiFe shield layers which also serve as conducting leads so that asense current can pass through. An illustration of this lead structureis not provided herein as it can be formed using different methods wellknown to practitioners of the art.

Referring next to FIG. 2, there is shown the structure of FIG. 1 awherein an alloy layer of Hf—Al (45) has been formed on the oxygensurfactant layer (30). In this, the second embodiment of the presentinvention, the Hf—Al layer will be naturally oxidized in place of the Allayer of the first embodiment. The Hf—Al layer is formed by firstdepositing a layer of Hf (41) to a thickness between approximately 1 and3 angstroms, with 2 angstroms being preferred and then depositing alayer of Al (43) to a thickness between approximately 4 and 5 angstromson the Hf layer. As a result of this deposition process, the Hf removesoxygen from the surfactant layer more reactively than the Al, since theheat of formation of HfO is higher than that of AlO, to form a stableHfO stoichiometry at the interface (46). In a manner similar to thatdescribed with reference to FIG. 1 c, the Hf—Al layer is then subjectedto a natural oxidation to complete the fully oxidized barrier layer. Thesubsequent formation of an upper electrode, which is a ferromagneticfree layer, and capping layer on the barrier layer proceed in the samemanner as that described in FIG. 1 d.

As is understood by a person skilled in the art, the preferredembodiment of the present invention is illustrative of the presentinvention rather than being limiting of the present invention. Revisionsand modifications may be made to processes, materials, structures, anddimensions through which is formed an MTJ read head with ultra-lowjunction resistance and high junction breakdown voltage, while stillproviding a method of forming such an MTJ read head in a manner which isin accord with the present invention as defined by the appended claims.

1. A method for forming a tunneling magnetoresistive (TMR) read headwith a magnetic tunnel junction (MTJ) configuration, wherein said readhead has an ultra-thin tunneling barrier layer with ultra-low resistanceand high breakdown voltage, comprising: providing a substrate; formingon said substrate a lower electrode of magnetic material, said lowerelectrode having an upper surface; forming an oxygen surfactant layer onsaid upper surface of said lower electrode; forming a tunneling barrierlayer on said upper surface of said lower electrode, said formationfurther comprising: forming a material layer on said surfactant layer,an interfacial portion of said material layer combining with oxygenwithin said surfactant layer and said oxygen forming within saidinterfacial portion an oxide with a uniform and stable stoichiometrythereby; oxidizing said material layer fully by a process of naturaloxidation to form, thereby, a tunneling barrier layer; forming an upperelectrode on said tunneling barrier layer; forming a capping layer onsaid upper electrode.
 2. The method of claim 1 wherein said lowerelectrode is a magnetically pinned layer whose formation furthercomprises: forming a seed layer on said substrate; forming amagnetically pinning layer on said seed layer; forming a syntheticantiferromagnetic (SyAF) layer on said pinning layer, said SyAF layercomprising first and second ferromagnetic layers separated by anantiferromagnetically coupling layer, wherein said first layer contactssaid pinning layer and said second ferromagnetic layer is a compositelayer having an upper surface on which can be advantageously formed anoxygen surfactant layer.
 3. The method of claim 2 wherein said compositelayer is formed as a layer of CoFe(10%) or CoFe(25%) of thicknessbetween approximately 15 and 20 angstroms on which is then formed alayer of NiFe(60%) of thickness between approximately 5 and 15 angstromsand wherein said NiFe has a lower binding energy with the oxygen in saidsurfactant layer than said CoFe.
 4. The method of claim 1 wherein saidoxygen surfactant layer is formed by a method comprising: placing saidsubstrate and lower electrode within an evacuated chamber; feeding saidchamber with a small amount of oxygen to obtain an oxygen pressuretherein of approximately 2×10⁻⁷ torr; exposing said upper surface ofsaid lower electrode to said oxygen for about 100 seconds, forming lessthan a monolayer of oxygen on said upper surface.
 5. The method of claim1 wherein said material layer is a layer of Al, formed to a thickness ofapproximately two atomic monolayers.
 6. The method of claim 1 whereinsaid material layer is a layer of Al, formed to a thickness betweenapproximately 5 and 6 angstroms.
 7. The method of claim 5 wherein saidAl layer combines with the oxygen in said surfactant layer to form astable Al₂O₃ stoichiometry at the interface between said layers.
 8. Themethod of claim 1 wherein said material layer is a composite layer ofHf—Al, wherein the Hf is deposited on said OSL as a layer of thicknessbetween approximately 1 and 3 angstroms and the Al is deposited on saidHf layer as a layer of thickness between approximately 4 and 5angstroms.
 9. The method of claim 8 wherein the Hf layer of saidcomposite layer combines with the oxygen in said surfactant layer toform a stable HfO stoichiometry.
 10. The method of claim 1 wherein saidmaterial layer is naturally and completely oxidized by a process furthercomprising: placing said substrate and said lower electrode having saidmaterial layer with said oxidized interfacial portion formed thereonwithin an evacuated chamber; feeding said chamber with an amount ofoxygen to obtain an oxygen pressure therein between approximately 0.1and 1 torr; exposing said material layer to said oxygen for a timebetween approximately 500 seconds and 200 seconds, wherein said longertime corresponds to said lower pressure and said shorter timecorresponds to said higher pressure.
 11. The method of claim 1 whereindiffusion of oxygen into said lower electrode during said process ofnatural oxidation is prevented by said interfacial portion of stablestoichiometry.
 12. The method of claim 6 wherein said Al layer combineswith the oxygen in said surfactant layer to form a stable Al₂O₃stoichiometry at the interface between said layers.